Araņya means forest. These were the books used by persons who have retired from the active life of householder gŗhasta and stayed in the forests to carry on their spiritual pursuits. These books mention yajňa, but emphasize their symbolism, especially the inner yajňa occurring in the subtle body of the human aspirant. These books naturally provide a bridge from the brāhmaņa books to the Upanishad books.
Typically these books constitute penultimate chapters of the brāhmaņa books. Sometimes they are separate books. For example Rigveda has a separate āraņyaka called as Aitareya āraņyaka which is associated with the corresponding aitareya brāhmaņa. Some opine that they are different chapters of the same book.
Shukla Yajurveda has no separate āraņyaka text, the last chapter of the brāhmaņa book of this Veda is the shatapatha brāhmaņa whose last chapter(s) constitute the famous Bŗhadāraņyaka Upanishad.
Again Chhāndogya brāhmaņa of Sāmaveda has no āraņyaka. It has the famous and massive Upanishad Chhāndogya.